Wrapper to allow SpanQuery objects participate in composite
single-field SpanQueries by 'lying' about their search field. That is,
the masked SpanQuery will function as normal,
but SpanQuery.getField() simply hands back the value supplied
in this class's constructor.

This can be used to support Queries like SpanNearQuery or
SpanOrQuery across different fields, which is not ordinarily
permitted.

This can be useful for denormalized relational data: for example, when
indexing a document with conceptually many 'children':

toString

Prints a query to a string, with field assumed to be the
default field and omitted.

The representation used is one that is supposed to be readable
by QueryParser. However,
there are the following limitations:

If the query was created by the parser, the printed
representation may not be exactly what was parsed. For example,
characters that need to be escaped will be represented without
the required backslash.

Some of the more complicated queries (e.g. span queries)
don't have a representation that can be parsed by QueryParser.